Final Summary Report Structural Integrity Assessment Bottom Ash Pond Cholla Power Plant Joseph City, Arizona

Submitted to Arizona Public Service Generation Engineering P.O. Box 53999 Phoenix, AZ 85072 Submitted by AECOM 7720 North 16 th Street Suite 100 Phoenix, AZ 85020 August 26, 2016 Final Summary Report Structural Integrity Assessment Cholla Power Plant Joseph City, Arizona Prepared for: Arizona Public Service AECOM Job No. 60445840 August 2016

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service i Table of Contents Certification Statement... 1 1 Introduction... 1-1 1.1 Report Purpose and Description... 1-1 1.2 EPA Regulatory Requirements... 1-1 1.3 Report Organization... 1-2 1.4 Facility Description... 1-2 2 Hazard Potential Classification... 2-1 2.1 Methodology and Design Criteria... 2-1 2.2 Hazard Potential Classification Results... 2-2 3 History of Construction... 3-1 3.1 Methodology... 3-1 3.2 Construction Summary... 3-1 4 Structural Stability Assessment... 4-1 4.1 Foundation and Abutments... 4-1 4.2 Slope Protection... 4-1 4.3 Dike Compaction... 4-2 4.4 Slope Vegetation... 4-2 4.5 Impoundment Capacity... 4-2 4.6 Hydraulic Structures... 4-3 4.7 Downstream Water Body... 4-3 4.8 Other Deficiencies... 4-3 4.9 Structural Stability Assessment Results... 4-3 5 Safety Factor Assessment... 5-1 5.1 Methodology and Design Criteria... 5-1 5.2 Critical Cross Section... 5-1 5.3 Subsurface Stratigraphy... 5-1 5.4 Material Properties... 5-2 5.5 Embankment Pore Pressure Distribution... 5-3 5.6 Embankment Loading Conditions... 5-3 5.7 Safety Factor Assessment Results... 5-5 6 Conclusions... 6-1 7 Limitations... 7-1 8 References... 8-1 August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service ii List of Appendices Appendix A. Appendix B. Historic Drawings Safety Factor Calculation List of Tables Table 3-1. History of Construction for Cholla... 3-2 Table 5-1. Selected Material Parameters Safety Factor Assessment... 5-3 Table 5-2. Range of Plasticity Index and Fines Content Values for Site Materials... 5-5 Table 5-3. Summary of Calculated Safety Factors... 5-5 List of Figures Figure 1-1. Site Vicinity Map... FIG-2 Figure 1-2. Monitored Instrumentation and Seep Location Map... FIG-3 Figure 3-1. Site Topography Map... FIG-4 Figure 3-2. Area Capacity Curve... FIG-5 Figure 5-1. Cross Section Locations for Safety Factor Assessment... FIG-6 August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service iii List of Acronyms ADWR Arizona Department of Water Resources APS Arizona Public Service CCR Coal Combustion Residual CFR Code of Federal Regulations EAP Emergency Action Plan EL Elevation EPA United States Environmental Protection Agency ft feet HDPE High Denisty Polyethylene HPC Hazard Potential Classification I-40 Interstate 40 pcf pounds per cubic foot psf pounds per square foot PI Plasticity Index PMF Probable Maximum Flood RCRA Resource Conservation and Recovery Act USCS Unified Soil Classification System USGS United States Geological Survey August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service CS-1 Certification Statement Certification Statement for: 40 CFR 257.73(a)(2)(ii) Initial Hazard Potential Classification for an Existing CCR Surface Impoundment 40 CFR 257.73(d)(3) Initial Structural Stability Assessment for an Existing CCR Surface Impoundment 40 CFR 257.73(e)(2) Initial Safety Factor Assessment for an Existing CCR Surface Impoundment CCR Unit: Arizona Public Service Company; Cholla Power Plant; I, Alexander Gourlay, being a Registered Professional Engineer in good standing in the State of New Mexico, do hereby certify, to the best of my knowledge, information, and belief, that the information contained in this certification has been prepared in accordance with the accepted practice of engineering. I certify, for the above-referenced CCR Unit, that the initial hazard potential classification, initial structural stability assessment, and initial safety factor assessment as included in the Structural Integrity Assessment Report dated August 17, 2016 was conducted in accordance with the requirements of 40 CFR 257.73. Alexander W. Gourlay, P.E. Printed Name August 26, 2016 Date August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 1-1 1 Introduction Arizona Public Service Company (APS) contracted URS Corporation, a wholly owned subsidiary of AECOM, to assist in the initial structural integrity assessment of the existing coal combustion residual (CCR) surface impoundments at the Cholla Power Plant in Joseph City, Arizona. Figure 1-1 shows the location of the CCR Impoundments at the Cholla Power Plant. This Summary Report documents the AECOM structural integrity assessment for the, Arizona Department of Water Resources (ADWR) Dam No. 09.27. Assessments of other CCR Impoundments at the Cholla Power Plant are presented in separate reports. 1.1 Report Purpose and Description The purpose of this report is to document the initial structural integrity assessment for the located at the Cholla Power Plant. The is an existing CCR surface impoundment owned and operated by APS that is regulated by the Arizona Department of Water Resources (ADWR). In 2015, the United States Environmental Protection Agency (EPA) finalized Federal Rule (Rule) 40 Code of Federal Regulations (CFR) 257.73 (EPA, 2015) regulating CCRs under subtitle D of the Resource Conservation and Recovery Act (RCRA). As part of this Rule, owners and operators of existing CCR surface impoundments must complete initial and periodic structural integrity assessments to document whether the CCR unit poses a reasonable probability of adverse effects on health and the environment. 1.2 EPA Regulatory Requirements Pursuant to Rule 40 CFR 257.73 (EPA, 2015), each existing CCR surface impoundment must have initial and periodic structural integrity assessments to evaluate whether the CCR unit poses a reasonable probability of adverse effects on health and the environment. The assessments must address the following elements: Periodic Hazard Potential Classification Assessment (40 CFR 257.73(a)(2)) - Document the hazard potential classification of each CCR unit as either a high hazard, significant hazard or low hazard potential CCR unit. Emergency Action Plan (EAP) (40 CFR 257.73(a)(3)) - Prepare and maintain a written EAP for high and significant hazard CCR units. The EAP must be evaluated at least every five years and, if necessary, updated and revised to maintain accurate information of current CCR unit conditions. The evaluation and certification of the EAP is provided in a separate report. In addition, the following elements must be addressed for CCR units, such as the, that have a height of five feet (ft) or more and a storage volume of 20 acre-ft or more, or have a height of 20 ft or more: History of Construction (40 CFR 257.73(c)(1)) - Compile a history of construction of the CCR unit including elements of operation, location, design, monitoring instrumentation, maintenance and repair, and historic structural instabilities. Periodic Structural Stability Assessment (40 CFR 257.73(d)) - Document whether the design, construction, operation and maintenance of the CCR unit is consistent with recognized and generally accepted good engineering practice for the maximum volume of CCR and CCR wastewater which can be impounded therein. Periodic Safety Factor Assessment (40 CFR 257.73(e)) - Document whether the calculated factors of safety for each CCR unit achieve minimum safety factors for the critical cross section of the embankment under long-term, maximum storage pool loading conditions, maximum surcharge loading conditions, seismic loading conditions, and post-earthquake loading conditions for dikes constructed of soils susceptible to liquefaction. August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 1-2 Existing CCR surface impoundments, such as the, are required to have an initial structural integrity assessment within 18 months of publication of the EPA Rule on April 17, 2015 and periodic assessments performed every five years thereafter. 1.3 Report Organization This Summary Report has been organized into the following sections: Report Section Applicable CFR 40 Part 257 Citation Section 1 Introduction Section 2 Hazard Potential Classification 257.73(a)(2) Periodic hazard classification assessments Section 3 History of Construction 257.73(c)(1) History of construction Section 4 Structural Stability Assessment 257.73(d) Periodic structural stability assessment Section 5 Safety Factor Assessment 257.73(e) Periodic safety factor assessment Section 6 Conclusions Section 7 Limitations Section 8 References Figures Appendix A Historic Drawings Appendix B Safety Factor Calculation 1.4 Facility Description The Cholla Power Plant is an electric generating station located in the town of Joseph City, Navajo County, Arizona. The station consists of four coal-fired units. Units 1, 2 (decommissioned), and 3 are owned by APS and Unit 4 is owned by PacifiCorp. CCR generated at the power plant are disposed of at two major surface impoundments located off-site; the Fly Ash Pond located about one-and-a-half miles east of the plant and the located about two miles north of the plant. Figure 1-1 shows the location of the Fly Ash Pond and in relation to the power plant. This assessment evaluates the structural integrity of the. The consists of a reservoir located in the southern portion of the pond, directly upstream of the dam and two coal combustion waste storage cells, the West Cell and the East Cell, located in the northern portion of the pond. The Bottom Ash Pond receives waste water from the Bottom Ash Transfer Sump that contains water and solids from the following sources: bottom ash overflow sump; bottom ash slurry from Units 1 through 4; Area 1, 2, and 3 Area Drainage Sumps; Units 1, 2, 3, and 4 Bottom Ash Hoppers; General Water Sump Liquids and Solids; Sedimentation Pond effluent; Units 1, 2, 3, and 4 Oil Water Separators; boiler cleaning waste; and water siphoned back from the. In addition, the following are discharged to the : scrubber sludge, stormwater, Units 3 and 4 Cooling Tower Basin Solids, seepage and stormwater from the Bottom Ash Monofill retention basins, General Water Sump Solids, Sedimentation Pond solids, WARP Solids, Flue Gas Desulfurization Wastes, and oil/water separator solids. The CCR wastes and other discharges are pumped to one of the two upstream waste storage cells, where the bottom ash is allowed to settle and the water is decanted to the reservoir for reuse at the power plant. At any given time, one of the waste storage cells is receiving bottom ash while the other is drained, excavated, and transported to a monofill north of the pond where the bottom ash is dry stacked. Excess water from the upstream cells are drained to the downstream reservoir via 12-inch pipes that outlet directly to the reservoir. The water is decanted from the reservoir for reuse at the power plant through a siphon system. August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 1-3 The has a total surface area of about 80 acres and a total storage capacity including solids and water of about 2,300 acre-ft when at its ADWR permitted maximum storage pool water level of EL 5,117.8 ft (ADWR, 1986). The impoundment is surrounded on its west and north sides by natural topography consisting of rock outcrops of mudstones, siltstones, and sandstones. On the south and east side, the impoundment is enclosed by the Dam, Arizona ADWR Dam No. 09.27, which spans the Tanner Wash. The has been classified under ADWR regulations as a high hazard impoundment due to probable loss of human life at the nearby U.S. Interstate 40 (I-40), Cholla Power Plant, freight railroad line, and downstream residences, in the event of a dam breach. The Dam is an earthen, zoned embankment dam consisting of a central clay core surrounded by an outer sand and gravel shell (random material zone). Construction began on the dam in 1976 and it started receiving CCR materials in 1978. In 1993 the dam crest was raised 3.3 ft when it became apparent that the storage volume of the pond was inadequate and the pond was filling faster than anticipated. In 1999 the impoundment was altered to its current configuration consisting of two upstream waste drainage cells and a downstream reservoir. The two waste cells are alternately dried and dredged to facilitate the removal of bottom ash from the sluice water which drains to the reservoir. By this procedure, the total storage volume in the pond remains relatively constant. The dam is approximately 4,200 ft in length with a maximum toe to crest height of 73 ft and crest width of 12 ft. The top of crest elevation is elevation (EL) 5,123.3 ft after the 1993 crest raise providing 5.5 ft of freeboard above the maximum permitted storage pool water level. Both the upstream and downstream slopes are inclined at a three horizontal to one vertical (3H:1V) angle except for the upper portion of the slopes constructed during the crest raise where the slopes are inclined at a 1.5H:1V vertical angle. Both upstream and downstream slopes are lined with riprap facing to prevent erosion. To limit seepage beneath the foundation, the central clay core of the Dam extends to bedrock at relatively shallow depths, less than 20 ft. In the center portion of the dam where the depth to bedrock is greater than 20 ft,, a slurry cutoff wall extends from the clay core to the bedrock or stiff clay. The Dam has no internal drain system; however, where seepage has been observed downstream of the dam, sumps have been installed to collect surface storm water and groundwater and return it to the pond. These include seepage collection systems for the West Abutment Seep, the Petroglyph Seep, the P-226 Seep, and the Tanner Wash Seep. The has no fixed intake or outlet water work structures. Sluiced bottom ash is pumped from the plant to the pond through a discharge line that runs up the right dam abutment, adjacent to the embankment and to a screening plant that scalps off some of the bottom ash solids for commercial use as light weight aggregate by the Salt River Materials Group. The processed slurry is then pumped to one of the two waste cells where the bottom ash settles and the excess water is decanted to the reservoir. Water levels within the pond are controlled by varying the pumping rate from the plant and seepage control system to balance with seepage, evaporation, and siphon system in the reservoir. The siphon system consists of three 12-inch diameter high density polyethylene (HDPE) pipes that float near the surface of the reservoir. The pipes were originally 8-inch in diameter but were replaced with the current 12-inch diameter pipes in the late 2000s. The siphon system pipes extend through the upper portion of the dam at a pipe invert elevation of about EL 5,120.5 ft and continue down the downstream face of the dam to a common valve chamber that combines the flow into a return pipe. The dam was constructed without an overflow spillway channel. To prevent overtopping during the design storm event, defined as the probable maximum flood (PMF), the pond was constructed to fully contain the storm runoff on top of the maximum permitted storage pool water level. This water level, defined as the maximum surcharge pool water level, is estimated at EL 5,119.3 ft based on an expected water level rise of 1.5 ft during the PMF (Dames & Moore, 1991). Piezometers, settlement monuments, flow measurement devices, and water level gauges are installed at the to monitor the performance of the dam. Measurements from the monitoring instruments are reviewed by AECOM and documented annually in a data report. Starting on October 19, 2015, the piezometer, survey monuments, V-notch weirs, and sumps are read at intervals not exceeding 30 days per the requirements of 40 CFR 257.83(a)(1)(iii). The locations of the piezometers, survey monuments, weir, and flow totalizers are shown on Figure 1-2. Inspections of the are performed by a qualified person at intervals not exceeding seven days. The inspections examine the for actual or potential conditions that could disrupt the operation or safety of the impoundment and documents the results of the inspection in the facility s operating record. In addition, a more detailed annual inspection is performed by a qualified professional engineer. The annual inspection includes a review of available information on the dam including the past year of monitoring data, a field inspection of the dam, abutment, and downstream toe and documentation of findings and recommendations in a dam safety inspection report. The most recent annual inspection of the was performed on October 16, 2015 (AECOM & APS, 2016). August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 2-1 2 Hazard Potential Classification This section summarizes the initial Hazard Potential Classification (HPC) for the. This initial HPC is intended to meet the requirement for periodic hazard potential classification assessment of existing CCR surface impoundments per Rule 40 CFR 257.73(a)(2). 2.1 Methodology and Design Criteria Per the Rule, the hazard potential classification provides an indication of the possible adverse incremental consequences that result from the release of water or stored contents due to failure or mis-operation of the CCR surface impoundment. The classification is based solely on the consequences of failure. As such, it is not dependent of the condition of the embankment or the likelihood of failure. Classifications per the Rule are separate from relevant and/or applicable federal, state or local dam safety regulatory standards, which may also include hazard classification definitions, and are not intended to substitute for other regulatory hazard potential classifications. Rule 40 CFR 257.53 defines three hazard potential classifications as follows: High hazard potential CCR surface impoundment A diked surface impoundment where failure or mis-operation will probably cause loss of human life. Significant hazard potential CCR surface impoundment A diked surface impoundment where failure or mis-operation results in no probable loss of human life, but can cause economic loss, environmental damage, disruption of lifeline facilities, or impact other concerns. Low hazard potential CCR surface impoundment A diked surface impoundment where failure or mis-operation results in no probable loss of life and low economic and/or environmental losses. Losses are principally limited to the surface impoundment s owner s property. The hazard potential of the was assessed qualitatively, per the above definitions. The qualitative assessment process is generally performed in a step-wise manner by first determining whether the pond could be classified as low hazard potential, based on immediately obvious factors such as proximity to property lines and/or surface water bodies. After determining that a structure does not meet the criteria for a Low Hazard Potential classification, the structure is assessed to determine whether it meets the criteria for High Hazard Potential. The potential for loss of life differentiates between high and significant hazard potential in the Final CCR Rule, therefore if the Dam does not meet the criteria for high hazard potential, it would be classified as a Significant Hazard Potential structure. The potential for downstream loss of life is assessed by reviewing land use in areas downstream (to the south) from the Dam, where inundation is likely in the event of a release. A dam break analysis and inundation mapping has been documented for the (Stantec, 2000). The inundation was reportedly mapped downstream in the Tanner Wash to the Joseph City Wash. Habitable structures reported in the inundation area included I-40, the Burlington Northern & Santa Fe Railroad, and the Cholla Power Plant (Stantec, 2000). The United States Geological Survey (USGS) 7.5-Minute Quadrangle topographic map of Joseph City, Arizona and associated digital orthoimage data (USGS, 2013) were used to review downstream areas for existing permanent and temporary land use. Permanent land uses include permanently inhabited dwellings and worksite areas that would likely contain workers on a daily basis (public utilities, power plants, water and sewage treatment plants, private industrial plants, sand and gravel plants, farm operations, fish hatcheries). Temporary land uses include primary roads, established campgrounds, or other recreational areas. August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 2-2 2.2 Hazard Potential Classification Results Inspection of the Dam and its immediate surrounding based on review of the USGS 7.5-Minute Quadrangle topographic map of Joseph City, AZ (USGS, 2013) and the dam break analysis report (Stantec, 2000) identifies that the is located approximately 2,000 ft upstream of I-40, a major east-west route of the Interstate Highway System. A catastrophic and unexpected failure of the Dam would likely inundate the travel lanes of I-40 and could result in loss of life. The is therefore classified as a High Hazard Potential CCR surface impoundment. August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 3-1 3 History of Construction This section summarizes the history of construction for the. This information is intended to meet the requirement for compilation of the history of construction for each CCR surface impoundment per Rule 40 CFR 257.73(c)(1). 3.1 Methodology AECOM reviewed available documents obtained from APS, the ADWR Document Repository, or in-house resources for information regarding the history of construction for the. Per the Rule, the compiled history of construction should include, to the extent feasible, the following information: Information identifying the CCR Unit, its purpose and the name and address of the owner/operator; The location of the CCR unit on the most recent USGS or other topographic map; Name and size of the watershed within which the CCR unit is located; A description of the physical and engineering properties of the foundation and abutment materials on which the CCR unit was constructed; A description of the type, size, and physical and engineering properties of each embankment zone; Provide detailed engineering drawings; A description of the type, purpose and location of existing instruments; Area-capacity curves for the CCR unit; A description of spillway and diversion design features; Construction specifications and provisions for surveillance, maintenance, and repair of the CCR unit; and Any record of knowledge of structural instability. 3.2 Construction Summary The history of construction dating back to the original construction that began in 1976 is summarized in Table 3-1 below. August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 3-2 Table 3-1. History of Construction for Cholla Item As-Constructed/ Current Comments Reference Document Name and Address of Owner Arizona Public Service Company (APS): P.O. Box 53999, Phoenix, Arizona 85072 --- --- State ID No. 09.27 --- ADWR License of Approval dated December 11, 1998 Size Classification Intermediate --- Hazard Classification High --- Annual CCR Impoundment and Landfill Inspection Report 2015 (AECOM & APS, 2016) Annual CCR Impoundment and Landfill Inspection Report 2015 (AECOM & APS, 2016) Construction Date Original: 1976 to 1977 Toe Drain System: 1979 Embankment Raise: 1993 Seepage Collection System: 1993 Impoundment Reconfiguration: 1999 --- Ash Pond Construction Memorandum (Ebasco, 1977) As-built Drawing No. G-44556 (Ebasco, 1977) Toe Drain System Drawing No. D-82671 (APS, 1991) Siphon System & Floating Pipeline As-built APS Drawing No. G566-S02 (APS, 1993) Location on USGS Quadrangle Map Statement of Purpose Joseph City Quadrangle: Section 13, Township 18 North, Range 19 East Bottom ash containment See Figure 3-1 Joseph City Quadrangle (USGS, 2013) Seepage and Foundation Studies: Volume I of II Engineering Report (Ebasco, 1975). Name of Watershed --- --- --- Size of Watershed (ac) 128 --- Flood Routing Report (Ebasco, 1976) Area Capacity Curve See Figure 3-2 --- Seepage and Foundation Studies: Volume I of II Engineering Report (Ebasco, 1975) Embankment Type Zoned earth fill dam consisting of a clay core and shell --- As-built Drawing No. G-44556 (Ebasco, 1977) Embankment Maximum Height (ft) 73 --- As-built Drawing No. G-556 (Ebasco, 1977) Siphon System & Floating Pipeline As-built APS Drawing No. G566-S02 (APS, 1993) Design Total Freeboard (ft) 5.5 --- Annual CCR Impoundment and Landfill Inspection Report 2015 (AECOM & APS, 2016) August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 3-3 Item As-Constructed/ Current Comments Reference Document Embankment Length (ft) 4,040 --- Annual CCR Impoundment and Landfill Inspection Report 2015 (AECOM & APS, 2016) Embankment Crest Elevation (ft) Original: 5,120 Modified: 5123.3 Embankment raised in 1993 As-built Drawing No. G-556 (Ebasco, 1977) Siphon System & Floating Pipeline As-built APS Drawing No. G566-S02 (APS, 1993) Embankment Crest Width (ft) Original: 16 Modified: 12 --- As-built Drawing No. G-44556 (Ebasco, 1977) Annual CCR Impoundment and Landfill Inspection Report 2015 (AECOM, 2016) Embankment Slopes Slope Protection Original 3H:1V (downstream & upstream) Raised Upper 3 ft: 1.5H:1V (downstream & upstream) Riprap --- As-built Drawing No. G-44556 (Ebasco, 1977) Annual CCR Impoundment and Landfill Inspection Report 2015 (AECOM & APS, 2016) Maximum Operating Storage Level (ft) 5117.8 Previous maximum storage levels were: 5,116 ft (1981); 5,114 ft (1984, 1986); 5,115 ft (1990, 1992); 5,118.6 ft (1993) ADWR License of Approval dated December 11, 1998 Storage Capacity (ac-ft) 2,300 --- Annual CCR Impoundment and Landfill Inspection Report 2015 (AECOM & APS, 2016) Surface Area (ac) 80 --- Annual CCR Impoundment and Landfill Inspection Report 2015 (AECOM & APS, 2016) Clay Core Properties Physical Properties The clay core consists of compacted sandy lean clay and sandy fat clay. --- Engineering Properties Moist Unit Weight = 120 pounds per cubic foot (pcf) Saturated Unit Weight = 125 pcf Effective Cohesion = 0 pounds per square foot (psf) Effective Friction Angle = 28 o Undrained strength ratio = 0.38 --- Seepage and Foundation Studies: Volume II of II Field and Laboratory Tests (Ebasco, 1975) Safety Inspection Report (Harza, 1987) Evaluation of Dam Embankment Crack (Dames & Moore, 1999) Shell (Random Zone) Properties August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 3-4 Item As-Constructed/ Current Comments Reference Document Physical Properties Engineering Properties The shell consists of compacted silty or clayey sand and sandy lean clay. Moist Unit Weight = 125 pcf Saturated Unit Weight = 130 pcf --- Effective Cohesion = 0 psf Effective Friction Angle = 33 o --- Foundation Conditions Seepage and Foundation Studies: Volume II of II Field and Laboratory Tests (Ebasco, 1975) Safety Inspection Report (Harza, 1987) Evaluation of Dam Embankment Crack (Dames & Moore, 1999) Physical Properties The embankment is generally founded on an engineered keyway consisting of the compacted clay core extending to competent bedrock. The exposed bedrock was cleaned and received grout treatment prior to placement of fill material. Where bedrock is deeper than 20 ft, a soil-bentonite cutoff wall extends through the alluvium to bedrock or stiff clay. The alluvium is a Quaternary age wash deposit consisting of unconsolidated clays, silts, and sands. The underlying bedrock consists of mudstone, siltstone, and sandstone associated with the Chinle and Moenkopi Formations. --- Seepage and Foundation Studies: Volume II of II Field and Laboratory Tests (Ebasco, 1975) Safety Inspection Report (Harza, 1987) Engineering Properties Alluvium: Moist Unit Weight = 120 pcf Saturated Unit Weight = 120 pcf Effective Cohesion = 0 psf Effective Friction Angle = 26 o Bedrock: Moist Unit Weight = 150 pcf Saturated Unit Weight = 150 pcf Effective Cohesion = 1,000 psf Effective Friction Angle = 65 o --- Engineering Properties (continued) Cutoff Wall: Moist Unit Weight = 106 pcf --- Seepage and Foundation Studies: Volume II of II Field and Laboratory Tests (Ebasco, 1975) August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 3-5 Item As-Constructed/ Current Comments Reference Document Saturated Unit Weight = 106 pcf Safety Inspection Report (Harza, 1987) Effective Cohesion = 0 psf Effective Friction Angle = 28 o Undrained Strength = 10 psf Abutment Conditions Physical Properties Engineering Properties The abutments consist of bedrock comprising mudstone, siltstone, and sandstone associated with the Chinle and Moenkopi Formation. The cut off wall that was part of the engineered foundation of the embankment was extended 350 ft beyond the end of the dam into the right abutment. Moist Unit Weight = 150 pcf Saturated Unit Weight = 150 pcf --- Effective Cohesion = 1,000 psf Effective Friction Angle = 65 o --- Seepage and Foundation Studies: Volume II of II Field and Laboratory Tests (Ebasco, 1975) Safety Inspection Report (Harza, 1987) Evaluation of Dam Embankment Crack (Dames & Moore, 1999) Spillway None The impoundment has sufficient storage volume above the maximum storage pool water level to store the IDF (PMF) and maintain at least four feet of freeboard. Annual CCR Impoundment and Landfill Inspection Report 2015 (AECOM & APS, 2016) Construction Specifications Clay Core: Fines content ranging from 50% to 100% No particle sizes greater than 3 inches Plasticity index ranging from 15 to 50 Fill lift thickness = 8 inches Minimum degree of compaction = 95% (standard Proctor) Test frequency = 60,000 ft 2 /test --- Ash Pond Construction Memorandum (Ebasco, 1977) Construction Specifications (continued) Shell (Random Zone): Maximum rock fraction greater than --- Ash Pond Construction Memorandum (Ebasco, 1977) August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 3-6 Item As-Constructed/ Current Comments Reference Document 3 inches = 10% Fill lift thickness = 12 inches Minimum degree of compaction = 100% (standard Proctor) Test frequency = 60,000 ft 2 /test Cutoff Wall: Preparation: o Minimum unit weight = 1.02 grams/cubic centimeter (g/cm 3 ) o Minimum viscosity = 30 secmarsh o Maximum filtration loss = 30 cm 3 o Minimum ph = 8 In Trench: o Unit weight range between 1.05 and 1.3 g/ cm 3 Backfill Mix at Trench: o Slump ranging between 3 and 6 inches o Percent passing 3/8-inch between 60 and 90% o Percent passing No. 20 sieve between 30 and 70% Fines content between 15 and 30% Detailed Drawings See Appendix A for drawings --- Original As-built (Ebasco, 1977) Ash Disposal Line Reroute (Ebasco, 1980) Crest restoration (APS, 1990) Siphon System & Floating Pipeline (APS, 1993) Seepage Interception System (APS, 1993) Existing Instrumentation August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 3-7 Item As-Constructed/ Current Comments Reference Document Type and Purpose of Instrumentation Open standpipe piezometers and wells for monitoring the phreatic levels in the embankment and foundation. Settlement monuments for monitoring movement of the embankment. Water level gauge for monitoring water level in reservoir. V-notch weir and seepage monitoring systems for measuring seepage rates. ---. Annual CCR Impoundment and Landfill Inspection Report 2015 (AECOM & APS, 2016) Location of Instrumentation Open standpipe piezometers and wells located in and around the embankment. Movement monuments located along the embankment crest. Water level gauge located in the reservoir. V-notch weir and seepage monitoring systems located along the downstream toe. See Figure 1-2 Annual CCR Impoundment and Landfill Inspection Report 2015 (AECOM & APS, 2016) Provisions for Surveillance, Maintenance and Repair Visual inspections of the dam by a qualified person on a frequency not exceeding seven days. Visual inspections of the dam conducted annually by a professional engineer. Phreatic level behavior from piezometric measurements and reservoir water level from gauge collected on an interval not exceeding 30 days. Embankment settlement using movement monuments survey data collected on an interval not exceeding 30 days. Seepage monitoring at the downstream toe collected on an interval not exceeding 30 days. --- Annual CCR Impoundment and Landfill Inspection Report 2015 (AECOM & APS, 2016) August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 3-8 Item As-Constructed/ Current Comments Reference Document Record of Structural Instability (See Section 4 for more details) Historic seepage at downstream toe Active seepage at the right abutment. Seepage areas near the downstream toe are identified as Toe Drain Seep, Petroglyph Seep, Tanner Wash Seep, and P-226 Seep. Notes: 1) Site elevations use National Geodetic Vertical Datum (NGVD) 1929 See Figure 1-2 for seepage areas. The seepage areas are captured and monitored by a seepage interceptor system near the downstream toe. Annual CCR Impoundment and Landfill Inspection Report 2015 (AECOM & APS, 2016) August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 4-1 4 Structural Stability Assessment This section summarizes the structural stability assessment for the. This information is intended to satisfy the requirement of Rule 40 CFR 257.73(d). 4.1 Foundation and Abutments Per the requirements of 40 CFR 257.73(d)(1)(i), existing CCR impoundments must be assessed for Stable foundations and abutments. The Dam is founded on alluvium overburden associated with the Tanner Wash with both abutments resting on bedrock consisting of mudstone, siltstone, and sandstone associated with the Chinle and Moenkopi Formations. Review of the as-built design drawings of the dam (Ebasco,1990) and construction inspection reports prepared by ADWR (formerly Arizona Water Commission) indicate a cut off trench was excavated at the abutments to extend the clay core to bedrock. When the depth to bedrock was greater than 20 ft, a soil-bentonite slurry cut-off wall was installed to the bedrock or to a stiff clay layer found about 60 to 70 ft below the original ground surface. In addition, an approximately 350 ft long slurry wall was installed beyond the right abutment to help control seepage through the Moenkopi bedrock formation. Review of the construction records indicates that where the cutoff trench was excavated to bedrock, loose rock was scaled from the foundation, dental concrete was applied to irregularities to create a relatively level surface, and a thin lift of wet cement tack coat was applied to the bedrock surface before placement of the clay core. For the shell of the dam, which is founded on alluvium overburden soils, the alluvium foundation was proof-compacted using a heavy dynamic compactor and surface stringers of sandy soils that crossed the dam foundation were removed. Several seepage locations have been observed downstream of the dam since the went into operation. These seeps are thought to occur due to a combination of normal flow through the embankment, discontinuities in the foundation near the groin of the abutment, and flow through gypsum seams in the Moenkopi Formation. Drain systems have been installed at most of the seepage locations, typically consisting of underground French drains connected to a collection sump. Four sumps and one weir have been installed at the following seeps: the P-226 Seep, the Petroglyph Seep, the Tanner Wash Seep, and the West Abutment Seep. The locations of the seeps are shown in Figure 1-2. Flow from the sumps and weir installed at the seeps are monitored and presented in the annual dam inspection reports. Flow rates ranging from 0 to 25 gallons per minute over the last ten years were measured at the sumps and weirs (AECOM & APS, 2016), indicating very low to moderate flow. The turbidity of the seep water observed at the sumps was also low. Both the low flow rate and the lack of turbidity indicate a low potential of internal erosion of the dam embankment or foundation. Review of the measured displacements of the survey monuments at the crest of the Dam, as presented in the 2015 annual dam inspection report (AECOM & APS, 2016), indicates settlements along the crest of the dam of three to eight inches and horizontal movements of three inches or less in the last ten years. Settlement rates appear relatively constant over the last ten years at about one quarter of an inch per year with little horizontal movement upstream or downstream. The relatively small settlement and horizontal movements measured at the Dam are an indication of stability in the dam foundation and abutments. 4.2 Slope Protection Per the requirements 40 CFR 257.73(d)(1)(ii), existing CCR impoundments must be assessed for Adequate slope protection to protect against surface erosion, wave action, and adverse effects of sudden drawdown. A review on the as-built drawing of the Dam (Ebasco, 1990), indicates the dam was constructed with a two foot thick layer of random rock fill (riprap) to protect the upstream and downstream slopes against erosion. No specifications for riprap size were shown on the drawings; however, visual observations performed during dam inspection suggest they are cobble to boulder sized. The 2015 annual dam inspection report (AECOM & APS, 2016) reported no significant erosion of the August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 4-2 dam slopes indicating the riprap slope protection is performing adequately. Based on the inspection report and experience with similar riprap slope protection designs, the has adequate slope protection to protect against surface erosion, wave action, and adverse effects of sudden drawdown. 4.3 Dike Compaction Per the requirements 40 CFR 257.73(d)(1)(iii), existing CCR impoundments must be assessed for Dikes mechanically compacted to a density sufficient to withstand the range of loading conditions in the CCR unit. Based on review of a memorandum summarizing construction of the Dam (Temchin, 1977), the dam (or dike) was constructed by placement of soils in mechanically compacted thin lifts of a foot or less. Construction control of the compaction process was maintained using a method procedure where the soil preparation, placement, watering, blading, final watering, rolling, and lift thickness are specified based on the results of fill pad testing conducted prior to start of earthwork (Ebasco, 1977). In addition to the method controls discussed above, quality control testing consisting of comparison of in-situ measurements of soil density to Standard Proctor maximum dry density, American Society for Testing and Materials D 698, was performed at intervals of once every 60,000 square ft of material placed. Results of quality control testing are summarized in Ebasco Drawing APS-2742-SK-CH-J12 (Temchin, 1977). The drawing indicates 368 tests were conducted on Clay Core materials with 357 of the tests measuring densities greater than 95 percent of the Standard Proctor maximum density and a mean percent compaction of all tests of 98.4 percent of the Standard Proctor maximum density. The drawing indicates 104 tests were conducted on the outer shell materials with 91 of the tests measuring densities greater than 100 percent of the Standard Proctor maximum density and a mean percent compaction of all tests of 101.2 percent of the Standard Proctor maximum density. Based on the compaction method described in the construction summary memorandum and the quality control test results presented in Drawing APS-2742-SK-CH-J13, the Dam has been mechanically compacted to a density sufficient to withstand the range of loading condition expected at the site. 4.4 Slope Vegetation Per the requirements 40 CFR 257.73(d)(1)(iv), existing CCR impoundments must be assessed for Vegetated slopes of dikes and surrounding areas, except for slopes which have an alternate form or forms of slope protection. Note that the United States Court of Appeals for the District of Columbia Circuit remanded with vacatur the phrase not to exceed a height of six inches above the slope of the dike from this subsection of the Rule. As noted in Section 4.2, the dam was constructed with a two foot thick layer of random rock fill (riprap) slope protection; therefore, the dam is excluded from the vegetated slope requirements since it uses an alternate form of slope protection. 4.5 Impoundment Capacity Per the requirements 40 CFR 257.73(d)(1)(v), existing CCR impoundments must be assessed for A single spillway or a combination of spillways configured as specified in paragraph (d)(1)(v)(a) of this sections. The combined capacity of all spillways must be designed, constructed, operated, and maintained to adequately manage flow during and following the peak discharge from the event specified in paragraph (d)(1)(v)(b) of this section. The Dam was constructed without a spillway or other water release structure. To manage flow during the design storm event, the has been designed, constructed, operated, and maintained with sufficient storage volume over and above the maximum permitted storage pool water level (EL 5,117.8 ft) to store the PMF storm water inflow at EL 5,119.3 ft and maintain four ft of freeboard; therefore, the impoundment is capable of adequately managing (containing) the flow during and following the peak discharge from the PMF event as required for high hazard potential CCR impoundments. August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 4-3 4.6 Hydraulic Structures Per the requirements 40 CFR 257.73(d)(1)(vi), existing CCR impoundments must be assessed for Hydraulic structures underlying the base of the CCR unit or passing through the dike of the CCR unit that maintain structural integrity and are free of significant distortion, bedding deficiencies, sedimentation, and debris which may negatively affect the operation of the hydraulic structures. Three 12-inch diameter HDPE pipes associated with the siphoning system which returns water back to the plant are the only hydraulic structures that penetrate the Dam embankment. Review of the as-built drawings of the siphon system (Ebasco, 1993) indicates the pipes are installed near the crest of the dam with penetration invert elevations of EL 5120.5 ft. The maximum surcharge pool water level, the highest water level anticipated within the pond, is at EL 5,119.3 ft a little over a foot below the pipe penetrations. Since it is not anticipated that the water level will rise to the elevation of the pipe penetration, the pipes are not expected to negatively impact the operation of the dam. Furthermore, since the pipes are buried at a relatively shallow depth beneath the crest, significant distortion or bending of the pipes would be readily apparent and can be easily repaired. 4.7 Downstream Water Body Per the requirements 40 CFR 257.73(d)(1)(vii), existing CCR impoundments must be assessed as follows For CCR units with downstream slope which can be inundated by the pool of an adjacent water body, such as a river, stream or lake, downstream slopes that maintain structural stability during low pool of the adjacent water body or sudden drawdown of the adjacent water body. No structural stability deficiencies are present associated with inundation of the downstream slope of the Dam by an adjacent body of water since no pool of water, such as a river, stream or lake, is present downstream of the dam which could inundate the downstream slope. 4.8 Other Issues No deficiencies were identified for the that could affect the structural stability of the impoundment. However, during the most recent dam inspection (AECOM & APS, 2016), observations of excessive vegetation consisting of small- to medium-sized desert brush and small animal burrows were noted along the slopes and crest of the Dam. APS work crews subsequently removed vegetation in the identified areas. Although both the vegetation and the animal burrows were not of sufficient size to cause concern for the stability or erosion of the embankment, failure to promptly identify and correct these issues could lead to eventual deterioration of the embankment slope. It is recommended, therefore, to continue inspection and maintenance activities of the impoundment to identify and correct minor issues in order to prevent progressive deterioration of the embankment. 4.9 Structural Stability Assessment Results AECOM did not identify any structural stability deficiencies that would affect the structural condition of the CCR Impoundment based on the documents provided and reviewed as part of this assessment. AECOM assesses that the design, construction, operation and maintenance of the Fly Ash Pond are consistent with recognized and generally accepted good engineering practice for the maximum volume of CCR and CCR wastewater which can be impounded therein. August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 5-1 5 Safety Factor Assessment This section summarizes the safety factor assessment for the. This assessment is intended to satisfy the requirement of Rule 40 CFR 257.73(e). 5.1 Methodology and Design Criteria Slope stability analyses were performed to document minimum factors of safety for loading conditions identified by 40 CFR 257.73(e) using the software program SLOPE/W (GEO-SLOPE International, 2012). The analyses were performed using Spencer s Method; a limit equilibrium method of slices that satisfies both force and moment equilibrium and incorporates the effects of interslice forces. The analyses incorporate strength and density properties and pore pressure distributions described in Sections 5.4 and 5.5. The slope stability models are presented in Appendix B. 5.2 Critical Cross Section Safety factors were calculated for two cross sections of the Dam selected to represent different embankment geometries, heights, and stratigraphic conditions to provide confidence that the critical cross section was identified. The critical cross section is the cross section that is anticipated to be most susceptible to structural failure for a given loading condition. The critical cross section thus represents a most-severe case. Section locations were selected based on variation in the embankment height, presence of cutoff trench/cutoff wall, and stratigraphic conditions. Subsurface soil profiles were developed using as-built drawings and historical borings reported by Ebasco (1975) and Harza (1987). The locations of the cross sections along the Dam are shown in Figure 5-1. The cross sections analyzed are: Cross Section 1: This cross section corresponds approximately to Section A as shown on Figure 5-1 and the as-built plan (Ebasco, 1990). This section represents the maximum section in areas where bedrock is shallow and, thus, includes an extension of the embankment clay core forming a cutoff trench that is keyed into bedrock. The embankment is approximately 73 ft high and the upstream and downstream slopes are at 3H:1V except for the top 3.3 ft of the dam where they are 1.5H:1V. The zoned embankment at this section consists of a sandy lean clay core with an outer clayey sand shell and the foundation consists of approximately 10 ft of alluvial overburden (clays, silts, and sands) overlying interbedded layers of mudstone, siltstone, and sandstone bedrock. The clay core extends to form a cutoff trench that is keyed into the top of bedrock. Approximately 28 ft of hydraulically-placed bottom ash is impounded behind the embankment at the Cross Section 1 location, based on comparison between pre-construction topographic survey data (Ebasco, 1975) and topographic survey data collected in 2014 (Cooper Aerial Surveys, 2014). Cross Section 2: This cross section corresponds approximately to Section H as shown on Figure 5-1 and the as-built plan (Ebasco, 1990). This section represents the maximum section in area of the deepest bedrock (85 ft below the ground surface). The section includes a cutoff slurry wall beneath the embankment clay core. The embankment is approximately 73 ft high and the upstream and downstream slopes are at 3H:1V except for the top 3.3 ft of the dam where they are 1.5H:1V. The zoned embankment at this section consists of a sandy lean clay core with an outer clayey sand shell and the foundation consists of approximately 85 ft of alluvial overburden (clays, silts, and sands) overlying interbedded layers of mudstone, siltstone, and sandstone bedrock. The slurry cutoff wall consists of a minimum two ft thick soil-bentonite wall that extends from the clay core of the dam to a layer of dense clay at about 15 ft above the bedrock. 5.3 Subsurface Stratigraphy Idealized models of subsurface stratigraphic conditions for each cross section were developed based on design drawings (Ebasco, 1990) and previous geotechnical site investigations (Ebasco, 1975, Harza, 1987, and Dames & Moore, 1999). The stratigraphic units described as follows were used to develop SLOPE/W models for each cross section. August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 5-2 Embankment Core: The zoned embankment includes a central impervious clay core with 1H:1V side slopes and a clay cap at the embankment crest. Fine-grained material was obtained from upstream borrow pits along the dam alignment and mechanically compacted in lifts to construct the clay core. The clay core soils consist predominately of Sandy Lean Clay (CL) with isolated zones of Sandy Fat Clay (CH) based on the Unified Soil Classification System (USCS). Embankment Shell (Random Zone): The zoned embankment includes a more pervious zone, or shell, that flanks the clay core to support and protect the impervious core. The shell provides stability against rapid drawdown (upstream shell) and drainage (downstream shell). Shell material was obtained from upstream borrow pits along the dam alignment and mechanically compacted in lifts. Shell soils consist predominately of Silty Sand (SM), Clayey Sand (SC), and Sandy Lean Clay (CL) based on the USCS. Alluvium: Alluvial deposits overlie the bedrock beneath the embankment and are the foundation bearing layer over most of the embankment alignment. The alluvium consists of a Quaternary Age, heterogeneous mixture of unconsolidated clays, silts, and sands deposited by flows in an unnamed tributary to Tanner Wash prior to the construction of the. Bedrock: Bedrock beneath the embankment consists of mudstones, siltstones, and sandstones of the Triassic-age Chinle and Moenkopi Formations. Slurry Cutoff Wall: A slurry cutoff wall was constructed using soil-bentonite slurry where the depth to bedrock is greater than 20 ft and extended into either the bedrock or dense clay soils. Bottom Ash: Bottom ash waste product from the power generating process is pumped from the plant to the and allowed to settle hydraulically in two coal combustion waste storage cells upstream of the dam. Excess water from the storage cells decants to a reservoir directly behind the dam. 5.4 Material Properties Material properties for soil, rock and embankment construction materials were developed based on an analysis and interpretation of historical geologic and geotechnical data presented in: Ebasco Services Inc., Arizona Public Services Cholla Generating Station Ash Disposal Sites Seepage and Foundation Studies: Volume I of II Engineering Report (Ebasco, 1975), Harza Engineering Company, Safety Inspection Report on Fly Ash Dam, Bottom Ash Dam, and Cooling Dike (Harza, 1987), and Dames & Moore, Interim Report, Geotechnical Investigation for Evaluation of Dam Embankment Crack, FAP Dam, Cholla Power Plant, Joseph City, Arizona (Dames & Moore, 1999). The material properties developed by the dam designers and subsequent investigators were assessed for reliability and applicability to this safety factor assessment. The design report (Ebasco, 1975) indicated that soil strength parameters were obtained from laboratory testing. Specific details of the soil strength property derivations used for the original design stability analyses were not provided in the design report. The Harza investigation (1987) included more detailed documentation of the laboratory testing, soil strength derivations, and stability analyses performed in 1987. The parameters developed by Harza were used in subsequent stability analyses performed by Dames & Moore (1991). AECOM assessed the historical soil strength data and parameters used by previous investigators and found the Harza (1987) data to be the most reliable and applicable to this safety factor assessment. The material properties selected for use in the slope stability analyses of the Dam are presented in Table 5-1. The drained strength values presented in Table 5-1 were taken from Harza (1987). The undrained strength value presented in Table 5-1 for the Embankment Core was derived by AECOM based on an interpretation of the Harza triaxial compression test data. Undrained strength properties were not needed for other material types for the safety factor calculations. Moist unit weight values used in this safety factor assessment were taken from Dames & Moore (1991); saturated unit weights were interpreted by AECOM based on the moist unit weights and material types reported by previous investigators. The bottom ash unit weight was selected by AECOM to be 75 pounds pcf based on engineering experience with similar materials. August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 5-3 Table 5-1. Selected Material Parameters Safety Factor Assessment Material Saturated Unit Weight, sat (pcf) Moist Unit Weight, m (pcf) Effective Strengths Cohesion, c (psf) Friction Angle, (degrees) Undrained Strength, S u (psf) Total Strengths Undrained Strength Ratio Embankment Core 125 120 0 28-0.38 Embankment Shell 130 125 0 33 - - Alluvium 120 120 0 26 - - Bedrock 150 150 1,000 65 - - Slurry Cutoff Wall 106 106 0 28 10 - Bottom Ash 75 75 0 25 - - 5.5 Embankment Pore Pressure Distribution Water levels have historically been measured quarterly, but beginning in October of 2015, they are now monitored on an interval not exceeding 30 days in the piezometers installed along or near the and reported annually in an inspection report (AECOM & APS, 2016). These data were considered to be the most reliable indicators of pore pressure distribution within the Dam embankment. The pore pressure distributions were estimated for each section using water level measurements obtained from: Section 1: piezometers W-227, B-200, B-201, and B-218; Section 2: piezometers W-305, B-203, B-204, and B-205 Piezometer locations are shown on Figure 1-2. Piezometer data were used, along with pond water level under steady-state, maximum storage pool conditions (ADWR, 1986 and ADWR, 1998), and pond water levels under maximum surcharge pool conditions (Ebasco, 1975 and Dames & Moore, 1991) to estimate pore pressure distributions with the embankment sections. Piezometer data are presented in the calculation in Appendix B. 5.6 Embankment Loading Conditions Per 40 CFR 257.73(e)(1)(i) through (iv), the following loading conditions were analyzed for each developed stability cross section: Long-term, maximum storage pool, Maximum surcharge pool, Seismic loading, and Liquefaction These loading conditions are described in the following sub-sections. Long-Term, Maximum Storage Pool: The maximum storage pool loading is the maximum water level that will be maintained for a sufficient length of time for steady-state seepage or hydrostatic conditions to develop within the embankment. This loading condition is evaluated to document whether the CCR surface impoundment can withstand a maximum expected pool elevation with full development of saturation in the embankment under long-term loading. August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 5-4 The long-term, maximum storage pool loading condition was evaluated using the permitted water level of the pond, as stated in the ADWR operating license for the dam. Since the dam has no outlet structure and relies on pumping rate from the plant, seepage, evaporation, and the siphon return system to control water levels, the maximum storage pool was set at the maximum ADWR-permitted water levels. For the, the safety factor was calculated for the long-term maximum storage pool at EL 5,117.8 ft (ADWR, 1998). Maximum Surcharge Pool: The maximum surcharge pool loading is the temporary rise in pool elevation above the maximum storage pool elevation to which the CCR surface impoundment could be subject under inflow design flood state. This loading condition is evaluated to document whether the downstream slope of the CCR surface impoundment embankment can withstand the short-term impact of a raised pool level. The maximum surcharge pool considers a temporary pool elevation that is higher than the maximum storage pool that persists for a length of time sufficient for steady-state seepage or hydrostatic conditions to fully develop within the embankment. The maximum surcharge pool loading condition was evaluated using the expected water level raise during the design PMF of 1.5 ft (Dames & Moore, 1991). For the, the safety factor was calculated for the maximum surcharge pool at EL 5,119.3 ft. Seismic Loading: Seismic loading is evaluated to document whether the embankment is capable of withstanding a design earthquake without damage to the foundation or embankment that would cause a discharge of contents. The seismic loading condition is assessed for a seismic loading event with a two percent probability of exceedance in 50 years, equivalent to a return period of approximately 2,500 years. A pseudo-static analysis was used to represent the seismic loading condition. The seismic response of soil embankments is incorporated into the analysis method by adding a horizontal force to simulate the seismic force acting on the embankment during an earthquake. The horizontal force is applied in the pseudo-static analyses through the addition of a seismic coefficient into the limit equilibrium calculations. The seismic coefficient was selected using the following procedure: 1. Determine the peak horizontal ground acceleration (PGA) generated in bedrock at the site by an earthquake having the 2 percent probability of exceedance in 50 years; 2. Select a Site Class, per International Building Code definitions, which incorporates the effects of seismic wave propagation through the top 100 ft of the soil profile above bedrock, and calculate the adjusted for Site Class effects, PGA M; 3. Calculate the maximum transverse acceleration at the crest of the embankment, PGA crest, using the PGA M from step 2; and 4. Adjust the PGA crest using the method developed by Makdisi and Seed (1977) to account for the variation of induced average acceleration with embankment depth to calculate the seismic coefficient. Each of these steps is discussed in more detail in the calculation presented in Appendix B. The maximum average acceleration for the potential sliding mass was incorporated into the pseudo-static safety factor analyses as the horizontal seismic coefficient equal to 0.13, corresponding to the calculated, adjusted PGA crest value. The water level in the for the seismic loading analysis was set to EL 5,117.8 ft to match the long-term, maximum storage pool. The Clay Core and Cutoff Wall materials were assigned total strengths because it is anticipated that they will behave in an undrained manner due to the relatively rapid loading induced during the seismic event and the relatively low hydraulic conductivity of these materials. All other materials used effective strength parameters. Liquefaction: The liquefaction factor of safety is evaluated for CCR embankments and foundation soils that are believed to be susceptible to liquefaction based on representative soil sampling and construction documentation or anecdotal evidence from personnel with knowledge of the CCR unit s construction., The liquefaction factor of safety is calculated to document whether the CCR unit would remain stable if the soils in the embankment and/or foundation experienced liquefaction. Post-construction geotechnical exploration of the Dam (Harza, 1987 and Dames & Moore, 1999) indicated the Clay Core (embankment) and Alluvium Overburden (foundation) materials have plasticity indexes and fine contents as August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 5-5 shown in Table 5-2. Data are not included in Table 5-2 for the Embankment Shell material due to the very limited amount of available geotechnical data. The Embankment Shell material was sourced from the Alluvium Overburden and is anticipated to have similar properties. Generally, the behavior of soils that have fines contents greater than 35 percent are dominated by the plasticity of the fines (Idriss and Boulanger 2008). Fines with Plasticity Index (PI) less the seven tend to behave more sand-like and are susceptible to soil liquefaction, while those with PI greater than seven tend to behave more clay-like and are not susceptible to liquefaction. The lowest measured value of PI for both the Clay Core and Alluvium Overburden is 12, indicating these soils would tend to behave in a clay-like manner during a seismic event and not be susceptible to soil liquefaction. Consequently, a liquefaction factor of safety analysis was not performed for this impoundment. Table 5-2. Range of Plasticity Index and Fines Content Values for Site Materials Material Plasticity Index, % Fines Contents, % Minimum Value Maximum Value Minimum Value Maximum Value Clay Core 12 39 48 88 Alluvium Overburden 12 17 30 54 5.7 Safety Factor Assessment Results Table 5-3 summarizes the results of the safety factor analysis for the Dam, for a more detailed discussion of the results see the safety factor calculation presented in Appendix B. Table 5-3. Summary of Calculated Safety Factors Loading Condition Required Safety Factor [1] Calculated Safety Factor Section 1 Section 2 Long-term, maximum storage pool 1.50 1.58 1.55 Maximum surcharge pool 1.40 1.56 1.53 Seismic 1.00 1.05 1.07 Notes: [1] From 40 CFR 257.73(e)(1)(i) through (iii) (EPA, 2015) The calculated factors of safety for the two critical cross sections along the Dam exceeded the required minimum values for the long-term, maximum storage pool; the maximum surcharge pool; and the seismic (pseudo-static) loading conditions. August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 6-1 6 Conclusions Based on the findings and results of the structural integrity assessment, AECOM provides the following conclusions for the at the Cholla Power Plant. The is classified as a High Hazard Potential CCR surface impoundment. The embankment is founded on stable foundations and abutments. Seepage is limited by a clay core that extends to the bedrock in shallow locations or a cutoff slurry wall where the depth to bedrock is greater than 20 ft. Downstream seeps are captured and monitored by drainage systems typically consisting of French drains connected to sumps. The embankment has adequate slope protection consisting of riprap on both the upstream and downstream slopes. Based on the available information and quality control test results, the Dam embankment was mechanically compacted to a density sufficient to withstand the range of loading conditions anticipated at the site. The impoundment is capable of adequately managing the flow during and following the peak discharge from the PMF event without a spillway or other water release structures because the pond has been designed, constructed, operated, and maintained with sufficient storage volume above the maximum storage pool water level to store the PMF inflow and maintain at least four feet of freeboard. Factors of safety greater than the minimum values required by the CCR Rule were calculated for two critical cross sections along the Dam for loading conditions associated with the maximum storage pool water level, maximum surcharge pool water level, and design level seismic event. The liquefaction factor of safety of the impoundment was not analyzed due to the low potential for soil liquefaction of the embankment and foundation soils as determined from index test results. Based on review of available records concerning the and the results of the stability analyses, no deficiencies were noted that would affect the structural condition of the dam. August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 7-1 7 Limitations This report is for the sole use of APS on this project only, and is not to be used for other projects. In the event that conclusions based upon the data obtained in this report are made by others, such conclusions are the responsibility of others. The Initial Structural Stability Assessment presented in this report was based on available information identified in Reference Section of the report that AECOM has relied on but not independently verified. Therefore, the Certification of Professional Opinion is limited to the information available to AECOM at the time the Assessment was performed in accordance with current practice and the standard of care. Standard of care is defined as the ordinary diligence exercised by fellow practitioners in this area performing the same services under similar circumstances during the same period. Professional judgments presented herein are primarily based on information from previous reports that were assumed to be accurate, knowledge of the site, and partly on our general experience with dam safety evaluations performed on other dams. No warranty or guarantee, either written or implied, is applicable to this work. The use of the words certification and/or certify in this document shall be interpreted and construed as a Statement of Professional Opinion and is not and shall not be interpreted or construed as a guarantee, warranty, or legal opinion. August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 8-1 8 References AECOM & Arizona Public Services Company (APS), 2016, Cholla Power Plant Fly Ash Pond,, Sedimentation Pond and Bottom Ash Monofill: Annual CCR Impoundment and Landfill Report 2015, January. Arizona Department of Water Resources (ADWR), 1986, License of Approval, Cholla Dam and Reservoir, State File No. 09.28, October 21. ADWR, 1998, License of Approval, Cholla Dam and Reservoir, State File No. 09.27, December 11. APS, 1991, Cholla S.E. Station: Bottom Ash Dam Toe Drain System, Drawing No. D-82671, Revision No. 2, July 26. APS, 2016, Cholla Power Plant Bottom and Fly Ash Ponds & Cooling Reservoir Basic Data Report. Cooper Aerial Surveys Co., 2014, Cholla Ash 2014: June 13, 2009 Topographic Mapping, Prepared for Arizona Public Service Company, Drawing No. 1, Sheet No. 1. Dames & Moore, 1991, Design Report for Cholla S.E.S. Units 1 4 Raising Bottom Ash Dam To Elevation 123.3 Feet MSL, Prepared for Arizona Public Service Company, July. Dames & Moore, 1999, Interim Report, Geotechnical Investigation for Evaluation of Dam Embankment Crack, FAP Dam, Cholla Power Plant, Joseph City, Arizona, Prepared for Arizona Public Service Company, D&M Job No. 02353-143-022, May 9. Ebasco Services Inc. (Ebasco), 1975, Arizona Public Services Cholla Generating Station Ash Disposal Sites Seepage and Foundation Studies: Volume I of II Engineering Report, Prepared for Arizona Public Service Company, August. Ebasco, 1976, Arizona Public Services Cholla Steam Electric Station, Flood Routing Studies for Bottom Ash and Fly Ash Ponds, May. Ebasco, 1977, Ash Disposal System: Sections & Details, Prepared for Arizona Public Service Company, Drawing No. G-44556, As-built, November 8. Ebasco, 1977, Arizona Public Services Cholla Steam Electric Station Units 2, 3, & 4 Ash Pond Construction, November 1977 Final Report, December 5. Ebasco, 1979, Ash Disposal System: Instrumentation, Prepared for Arizona Public Service Company, Drawing No. SK-CH-J21, Revision No. 4, March 7. Ebasco, 1990, Ash Disposal System: Sections & Details, Prepared for Arizona Public Service Company, Drawing No. G-44556, Revision No. 12 (Crest Restoration 1990), August 28. Ebasco, 1993, Ash Disposal System: Siphon System & Floating Pipeline, Prepared for Arizona Public Service Company, Drawing No. G-44556 Sheet 2 (APS Drawing No. G-556-A02), As-built, February 26. GEOSLOPE International, 2012, GeoStudio 2012, Version 8.15.4.11512, August 2015 Release. Harza Engineering Company (Harza), 1987, Cholla S.G.S., Safety Inspection Report on Fly Ash Dam, Bottom Ash Dam, and Cooling Dike, Prepared for Arizona Public Service Company, February 27. Idriss, I.M. and Boulanger, R.W., 2008, Soil Liquefaction During Earthquakes, Earthquake Engineering Research Institute, Monograph No MNO-12. August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service 8-2 Makdisi, F.I. and Seed, H.B., 1977, A Simplified Procedure for Estimating Earthquake-Induced Deformations in Dams and Embankments, Report No. UCB/EERC-77/19, University of California, Berkeley, August. Stantec Consulting Inc (Stantec), 2000, APS Cholla Dambreak Study, Prepared for Arizona Public Service Company, November. Temchin, M., 1977, Arizona Public Service Company Cholla Steam Electric Station Units 2, 3, & 4 Ash Pond Construction November 1977 Final Report, Memorandum to G.W. Strengham of Arizona Public Service, Ebasco Services Inc., December 5. United States Environmental Protection Agency (EPA), 2015, 40 CFR Parts 257 and 261 Hazardous and Solid Waste Management System; Disposal of Coal Combustion Residuals from Electric Utilities; Final Rule, Federal Register Vol. 80, No. 74, April 17. United States Geological Survey (USGS), 2013, 7.5-Minute Series Joseph City, AZ Quadrangle Map August 2016 AECOM Job No. 60445840

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service FIG-1 Figures August 2016 AECOM Job No. 60445840

AECOM Final Summary Report FIG-2 Last saved by: ALEISA_KRUG(2016-06-16) Last Plotted: 2016-06-16 Filename: P:\ARIZONA PUBLIC SERVICE\60445840_APS_CHOLLA_STRUCTURAL_INTEGRITY\CADD FILES\FIGURES\FIGURE 1-1_SITE VICINITY MAP.DWG BOTTOM ASH POND CHOLLA POWER PLANT FLY ASH POND CHOLLA POWER PLANT STRUCTURAL INTEGRITY REPORT ARIZONA PUBLIC SERVICE Project No. 60445840 SITE VICINITY MAP FIGURE 1-1

AECOM Final Summary Report FIG-3 Last saved by: ALEISA_KRUG(2016-06-16) Last Plotted: 2016-06-16 Filename: P:\ARIZONA PUBLIC SERVICE\60445840_APS_CHOLLA_STRUCTURAL_INTEGRITY\CADD FILES\FIGURES\FIGURE 1-2_INSTRUMENT LOCATIONS.DWG PIEZOMETER LOCATIONS ARE APPROXIMATE B-217 B-216 B-214 B-215 B-213 B-212 B-211 B-210 P-226 SEEP B-218 W-227 B-204 B-209 B-208B TANNER WASH SEEP B-200 WEST ABUTMENT WEIR B-201 B-202 B-207 B-225 PETROGLYPH SEEP TOE DRAIN SEEP B-206 CHOLLA POWER PLANT STRUCTURAL INTEGRITY REPORT ARIZONA PUBLIC SERVICE Project No. 60445840 BOTTOM ASH POND MONITORED INSTRUMENTATION AND SEEP LOCATION MAP FIGURE 1-2

AECOM Final Summary Report FIG-4 Last saved by: ALEISA_KRUG(2016-06-16) Last Plotted: 2016-06-16 Filename: P:\ARIZONA PUBLIC SERVICE\60445840_APS_CHOLLA_STRUCTURAL_INTEGRITY\CADD FILES\FIGURES\FIGURE 4-1_SITE TOPO MAP.DWG BOTTOM ASH POND CHOLLA POWER PLANT FLY ASH POND CHOLLA POWER PLANT STRUCTURAL INTEGRITY REPORT ARIZONA PUBLIC SERVICE SITE TOPOGRAPHIC MAP Project No. 60445840 FIGURE 3-1

AECOM Final Summary Report FIG-5 Last saved by: ALEISA_KRUG(2016-06-16) Last Plotted: 2016-06-16 Filename: P:\ARIZONA PUBLIC SERVICE\60445840_APS_CHOLLA_STRUCTURAL_INTEGRITY\CADD FILES\FIGURES\FIGURE 4-2_AREA CAPACITY CURVE.DWG CHOLLA POWER PLANT STRUCTURAL INTEGRITY REPORT ARIZONA PUBLIC SERVICE AREA CAPACITY CURVE Project No. 60445840 FIGURE 3-2

AECOM Final Summary Report FIG-6 Last saved by: ALEISA_KRUG(2016-08-10) Last Plotted: 2016-08-11 Filename: P:\ARIZONA PUBLIC SERVICE\60445840_APS_CHOLLA_STRUCTURAL_INTEGRITY\CADD FILES\FIGURES\FIGURE 5-1_CROSS SECTION LOCATIONS SFA - BAP.DWG 0+00 1+00 2+00 3+00 4+00 5+00 6+0 ANSI A 8.5" x 11" B BOTTOM ASH POND (BAP) SECTION 1 SECTION 2 FOUR CORNERS POWER PLANT STRUCTURAL INTEGRITY REPORT ARIZONA PUBLIC SERVICE CROSS SECTION LOCATIONS SAFETY FACTOR ASSESSMENT Project No. 60445840 FIGURE 5-1

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service A-1 Appendix A. Historic Drawings August 2016 AECOM Job No. 60445840

AECOM Final Summary Report A-2 ORIGINAL AS-BUILT DRAWINGS (Ebasco, 1977)

AECOM Final Summary Report A-3

AECOM Final Summary Report A-4

AECOM Final Summary Report A-5

AECOM Final Summary Report A-6

AECOM Final Summary Report A-7

AECOM Final Summary Report A-8

AECOM Final Summary Report A-9 REROUTE 12 DIAMETER HDPE PIPE DRAWINGS (APS, 1980)

AECOM Final Summary Report A-10

AECOM Final Summary Report A-11

AECOM Final Summary Report A-12 CREST RESTORATION AS-BUILT (Ebasco, 1990)

AECOM Final Summary Report A-13

AECOM Final Summary Report A-14

AECOM Final Summary Report A-15

AECOM Final Summary Report A-16 TOE DRAIN SYSTEM (APS, Rev. 1991)

AECOM Final Summary Report A-17

AECOM Final Summary Report A-18 AS-BUILT SIPHON SYSTEM & FLOATING PIPE (Ebasco, 1993)

AECOM Final Summary Report A-19

AECOM Final Summary Report A-20 SEEPAGE INTERCEPT SYSTEM DRAWINGS (APS, 1993)

AECOM Final Summary Report A-21

AECOM Final Summary Report A-22

AECOM Final Summary Report A-23

AECOM Final Summary Report Structural Integrity Assessment Cholla Power Plant Arizona Public Service B-1 Appendix B. Safety Factor Calculation

AECOM Final Summary Report B-2

AECOM Final Summary Report B-3

AECOM Final Summary Report B-4 DESIGN CALCULATION Calculation Title: Project Title: Project No: Date: Page No: Safety Factor Assessment APS Cholla Structural Integrity Assessment TABLE OF CONTENTS 60445840 4/13/16 Page 1 of 15 1 Introduction... 2 2 Analysis Criteria... 2 3 Analysis Inputs... 2 4 Assumptions... 3 5 Stability Analysis... 3 5.1 Critical Stability Cross Sections... 4 5.2 Material Properties... 5 5.3 Embankment Pore Pressure Distribution... 6 5.4 Embankment Loading Conditions... 6 6 Analysis Results and Conclusions... 13 7 References... 14 8 Attachments... 15 Figures Figure 1 Slope Stability Cross Section and Piezometer Locations Along the BAP Dam Figure 2 Table 1615.1.1Site Class Definitions (IBC, 2003) Figure 3 Table 11.8-1 (NEHRP, 2009) Figure 4 Variations of Peak Transverse Crest Acceleration v. Peak Transverse Base Acceleration Based on Holzer (1998) Figure 5 Variation of Maximum Acceleration Ratio with Depth of Sliding Mass after Makdisi and Seed (1977) Tables Table 1 Table 2 Table 3 Material Properties for the BAP Dam Safety Factor Analyses Range of Plasticity Index and Fines Content Values for Site Materials Safety Factor Results for the BAP Dam

AECOM Final Summary Report B-5 DESIGN CALCULATION Calculation Title: Project Title: Project No: Date: Page No: Safety Factor Assessment 1 INTRODUCTION APS Cholla Structural Integrity Assessment 60445840 4/13/16 Page 2 of 15 The purpose of this calculation is to perform limit equilibrium slope stability analyses to assess the stability of the existing Coal Combustion Residual (CCR) surface impoundment dam Bottom Ash Pond (BAP) Dam, ADWR Dam #09.27,at Arizona Public Service (APS) s Cholla Power Plant near Joseph City, AZ. 2 ANALYSIS CRITERIA The analyses were performed to meet the regulations set forth in the United States Environmental Protection Agency (EPA) 40 CFR Part 257.73(e) Structural Integrity Criteria for Existing CCR Surface Impoundments (EPA 2015). The code requires safety factor assessments for units containing CCRs. The safety factors for various embankment loading and tailwater conditions must meet the values outlined. For the BAP Dam, the following safety factors must be met: Long-term, maximum storage pool FS = 1.50; Maximum surcharge pool FS = 1.40; Seismic loading FS = 1.00; and Liquefaction loading FS = 1.20 (only for sites with liquefiable soils). 3 ANALYSIS INPUTS The following inputs were used in the analysis: Surface profiles were developed from 2009 elevation contour drawings of the BAP Dam and surrounding terrain (Cooper Aerial Surveys Co. 2014). Subsurface stratigraphies were developed from as-built cross section drawings of the BAP Dam (Ebasco 1990). Material properties used in the model were developed in a separate calculation (AECOM 2016). Pore pressure distribution within the dam was developed from interpretation of water level readings for piezometers installed at the dam and surrounding area. Water level measurements are presented in the annual dam inspection report (APS 2016).

AECOM Final Summary Report B-6 DESIGN CALCULATION Calculation Title: Project Title: Project No: Date: Page No: Safety Factor Assessment APS Cholla Structural Integrity Assessment 60445840 4/13/16 Page 3 of 15 The maximum storage pool water level of the CCR Pond was based on the maximum permissible water level stated in the permitting license for the BAP (ADWR 1998). The surcharge pool water level of the CCR Pond was developed based on estimated water level for the design probable maximum flood (PMF) of the BAP (D&M 1991). The seismic loading for the BAP dam was developed from the deaggregated seismic hazard at the site based on the 2008 United States Geological Survey (USGS) National Earthquake Hazards Reduction Program (NEHRP) Provisions (USGS 2008). The slope stability analyses were performed using the software program SLOPE/W, commercially available through GEO-SLOPE International, Ltd. (GEO-SLOPE International 2012). 4 ASSUMPTIONS The following assumptions were used in the analysis: No bathymetry data was available for the BAP; therefore, the upstream slope of the BAP is assumed to be 3H:1V (horizontal:vertical) below the reservoir water level with negligible accumulation of bottom ash deposits. This slope angle is based on the as-built BAP Dam cross section drawings (Ebasco 1990). The surface profile for the site was developed based on the most recent topographic survey available, from June of 2009. It is assumed that the surface topography shown in this survey is sufficiently representative of the current topography so as not to produce significant differences in the estimated safety factors. This seems reasonable since there have been no significant alterations to the BAP Dam or the immediate surrounding areas since the survey was conducted, except for additional accumulation of bottom ash within the impoundment. The divider dikes associated with the waste coal combustion waste storage cells, located upstream of the BAP Dam, are internal dividers within the CCR surface impoundment and are not relied upon to maintain containment of the CCR. Consequently, the stability of the divider dikes is not analyzed in this calculation. 5 STABILITY ANALYSIS Slope stability analyses were performed to document minimum factors of safety for loading conditions identified by 40 CFR Section 257.73(e) using the software program SLOPE/W (GEO- SLOPE International, Ltd. 2012). The analyses were performed using Spencer s Method, a limit

AECOM Final Summary Report B-7 DESIGN CALCULATION Calculation Title: Project Title: Project No: Date: Page No: Safety Factor Assessment APS Cholla Structural Integrity Assessment 60445840 4/13/16 Page 4 of 15 equilibrium method of slices that satisfies both force and moment equilibrium in addition to incorporating the effects of interslice forces. 5.1 Critical Stability Cross Sections Factors of safety were calculated for critical cross-sections of the BAP Dam. The critical cross section is the cross section that is anticipated to be most susceptible to structural failure for a given loading condition. The critical cross section thus represents a most-severe case. Section locations were selected based on variation in the embankment height and stratigraphic conditions to represent the most-severe case. The safety factor assessments were performed for two cross-sections along the BAP Dam: BAP Dam Cross Sections Figure 1. Slope Stability Cross Section and Piezometer Locations Along the BAP Dam BAP Cross Section 1: Cross Section 1 at the BAP was located near the left abutment of the dam, near piezometers W-227, B-200, and B-201. At this location, the dam intersects a rock outcropping forming the left abutment along its upstream slope. Consequently the upstream and downstream slope heights are considerably different at approximately 31 ft versus 73 ft, respectively although both slope angles are about 3H:1V. The dam at this cross section consists of a

AECOM Final Summary Report B-8 DESIGN CALCULATION Calculation Title: Project Title: Project No: Date: Page No: Safety Factor Assessment APS Cholla Structural Integrity Assessment 60445840 4/13/16 Page 5 of 15 sandy lean clay core with an outer clayey sand shell. The dam lies on a foundation of alluvial overburden consisting of clays, silts, and sands; overlying bedrock consisting of mudstones, siltstones, and sandstones. The depth to bedrock is approximately 10 ft bgs. A cutoff trench filled with compacted clay extends from the clay core down to the bedrock and is used to control seepage beneath the dam. The upstream slope of the dam is confined by up to about 28 ft of hydraulically-placed bottom ash based on comparison between initial topographic surveys of the area (Ebasco 1975) and more recent surveys (Cooper Aerial Surveys 2014). BAP Cross Section 2: Cross Section 2 at the BAP was located near the center of the southern portion of the dam near piezometers W-305, B-203, B-204, and B-205. At this location, the dam is approximately 73 ft in height from EL 5,050 ft at the downstream toe to 5,123.3 ft at the crest; with upstream and downstream slope angles of about 3H:1V. Similar to Cross Section 1, the dam consists of a sandy lean clay core with an outer clayey sand shell. At this location the depth to bedrock is greatest along the dam at approximately 85 ft bgs. A cement-bentonite cutoff wall extends from the clay core of the dam to a layer of dense clay at about 15 ft above the bedrock and is used to control seepage beneath the dam. Water from the upstream coal combustion waste storage cells drains into a reservoir that lies upstream of the dam. 5.2 Material Properties A material properties calculation package was prepared to present the methods and information supporting the parameter selection for the materials at the BAP Dam (AECOM 2016). The material properties identified in the calculation and used in the slope stability analyses are presented in Table 1 below. Table 1. Material Properties for the BAP Dam Safety Factor Analyses Sat. Unit Moist Unit Drained Strengths Undrained Strengths Material Weight, sat (pcf) Weight, m (pcf) Cohesion, c (psf) Friction Angle, (degrees) Undrained Strength, S u (psf) Undrained Strength Ratio Clay Core 125 120 0 28-0.38 Shell 130 125 0 33 - - Alluvium 120 120 0 26 - - Bedrock 150 150 1,000 65 - - Cutoff Wall 106 106 0 28 10 - Bottom Ash 75 75 0 25 - -

AECOM Final Summary Report B-9 DESIGN CALCULATION Calculation Title: Project Title: Project No: Date: Page No: Safety Factor Assessment APS Cholla Structural Integrity Assessment 5.3 Embankment Pore Pressure Distribution 60445840 4/13/16 Page 6 of 15 Based on guidance from the EPA Regulations (EPA 2015), pore-water pressures are estimated from the most reliable of the following: 1) Field measurements of pore pressures in existing slopes; 2) past experience and judgment of the Engineer; 3) hydrostatic pressures calculated for the no-flow condition; or 4) steady-state seepage analysis using flow nets or finite element analyses. For the BAP Dam analysis, the pore pressure distribution was assigned using water level readings obtained from piezometers located near the stability cross sections (APS 2016). This distribution was adjusted based on engineering judgement to correspond with pond water level under steady-state, maximum storage pool conditions (ADWR 1998), and pond water level under maximum surcharge pool conditions (D&M 1991). The piezometers used to estimate the pore water pressure within the dam cross sections are shown in Figures 1. The BAP (upstream) water level under maximum storage pool condition was based on the permitted water level of the ponds as stated in the ADWR operating license for the dam. Since the dam has no outlet work structures and rely on pumping rate from plant, seepage, evaporation, and a siphon system to control water levels, the maximum storage pool was set at the maximum permitted water levels. For the BAP this is EL 5,117.8 ft (ADWR 1998). The surcharge pool level is based on the expected water level raise during the design PMF and is EL 5,119.3 ft for the BAP (D&M 1991). 5.4 Embankment Loading Conditions Per 40 CFR Section 257.73(e), the following loading conditions were considered for each selected stability cross section: Long-term, maximum storage pool; Maximum surcharge pool; Seismic loading; and Liquefaction. These loading conditions are described below. Long-Term, Maximum Storage Pool The maximum storage pool loading is the maximum water level that can be maintained that will result in the full development of a steady-state seepage condition. This loading condition is evaluated to document whether the CCR surface impoundments can withstand the maximum expected pool elevation with full development of saturation in the embankment under longterm loading. The maximum storage pool considers a pool elevation in the CCR unit that is equivalent to the maximum permitted water levels using shear strengths expressed as effective stress with pore water pressures that correspond to the long-term condition.

AECOM Final Summary Report B-10 DESIGN CALCULATION Calculation Title: Project Title: Project No: Date: Page No: Safety Factor Assessment APS Cholla Structural Integrity Assessment 60445840 4/13/16 Page 7 of 15 For this analysis, the long-term, maximum storage pool in the BAP was set at EL 5,117.8 ft. Since the piezometric conditions within the dam are at steady-state flow, drained material strengths were used in the analysis. Maximum Surcharge Pool The maximum surcharge pool loading is the temporary rise in pool elevation above the maximum storage pool elevation for which the CCR surface impoundment is normally subject under the inflow design flood state. This loading condition is evaluated to document whether the CCR surface impoundments can withstand a short-term impact of a raised pool level on the stability of the downstream slope. The maximum surcharge pool considers a temporary pool elevation that is higher than the maximum storage pool assuming that it persists for a length of time sufficient for steady-state seepage or hydrostatic conditions to fully develop within the embankment. For this analysis, the maximum surcharge pool in the BAP was set at EL 5,119.3 ft. Since the piezometric conditions within the dam are at steady-state flow for this loading condition, drained material strengths were used in the analysis. Seismic Loading Seismic loading was evaluated to document whether the CCR surface impoundments are capable of withstanding a design earthquake without damage to the foundation or embankment that would cause a discharge of its contents. The seismic loading is assessed under seismic loading conditions for a seismic loading event with a 2% probability of exceedance in 50 years, equivalent to a return period of approximately 2,500 years. A pseudostatic analysis was used to represent the seismic loading. The peak horizontal bedrock acceleration for a site classification of B Rock based on the USGS 2008 NEHRP seismic hazard map with a 2% probability of exceedance in 50 years is 0.0807g as presented in Attachment A (USGS 2008). Based on previous site explorations, a sit classification of D Stiff Soil was assigned to the site as illustrated in Table 1615.1.1 from the IBC (2003) shown in Figure 2.

AECOM Final Summary Report B-11 DESIGN CALCULATION Calculation Title: Project Title: Project No: Date: Page No: Safety Factor Assessment APS Cholla Structural Integrity Assessment 60445840 4/13/16 Page 8 of 15 Figure 2. Table 161.1.1 Site Class Definitions (IBC 2003) The PGA at the ground surface for Site Class D, or PGA M, was determined by amplifying the PGA for rock (Site Class B) using the following equation presented in NEHRP, 2009: PGA M = F PGA (PGA) PGA M = 1.6(0.0807g) PGA M = 0.129g Where: PGA M = Maximum considered earthquake geometric mean peak ground acceleration adjusted for Site Class effects PGA = Mapped maximum considered earthquake geometric mean peak ground acceleration F PGA = Site coefficient from Table 11.8-1 (Figure 3)

AECOM Final Summary Report B-12 DESIGN CALCULATION Calculation Title: Project Title: Project No: Date: Page No: Safety Factor Assessment APS Cholla Structural Integrity Assessment 60445840 4/13/16 Page 9 of 15 Figure 3. Table 11.8-1 Site Coefficient F PGA (NEHRP 2009) The PGA at the ground surface for Site Class D (PGA M ) was then used to estimate the peak transverse acceleration at the crest of the embankment, PGA crest = 0.307g, as shown on Figure 4 and based on variations in recorded peak crest accelerations versus those recorded at the base of earth and rock fill dams by Idriss (2015) and on recorded values for Loma Prieta, and other earthquakes, by Holzer (USGS, 1998).